1. Field of the Invention
The present invention relates to a cathode ray tube having a glass bulb which is mainly used for TVs.
2. Discussion of Background
As shown in FIG. 1, a cathode ray tube 1 for TVs comprises a glass bulb 2 which is basically constituted by a panel portion 3 for displaying a picture image, a funnel portion 4 on which a deflection coil is mounted and a neck portion 5 for receiving an electron gun 17.
In FIG. 1, reference numeral 6 designates a panel skirt portion, numeral 7 designates a panel face portion for displaying a picture image, numeral 8 designates an implosion-proof reinforcing band for providing strength, numeral 10 designates a sealing portion at which the panel portion 3 and the funnel portion 4 are sealed with solder glass or the like, numeral 12 designates a fluorescent film for emitting light by the excitation of irradiated electron beams, numeral 13 designates an aluminum film for reflecting light to the outside of a screen, numeral 14 designates a shadow mask for defining the position of irradiation of electron beams, numeral 15 designates a stud pin for fixing the shadow mask 14 to the inside surface of the panel skirt portion 6, and numeral 16 designates a inner conductive coating which prevents the shadow mask 14 from being charged by the electron beams and which leads electric charges to the outside. A symbol A indicates a tube axis which connects the center axis of the neck portion 5 and the center of the panel portion 3.
Since an atmospheric pressure is applied to the outer surface of the glass bulb for a cathode ray tube, which is used as a vacuum vessel, a stress (hereinbelow, referred to as a vacuum stress) is produced. The glass bulb has an asymmetric shape unlike a spherical shape, and accordingly, there are a region of tensile stress (a sign of +) and a region of compressive stress (a sign of -) in a relatively broad area on the glass bulb surface as shown in FIG. 2. In FIG. 2, a symbol .sigma..sub.R represents a component of stress along the paper surface and a symbol .sigma..sub.T represents a component of stress perpendicular to the paper surface. The numerical values described near the distribution lines of stress represent the values of stress at these positions.
There is a two-dimensional distribution of stress in the front surface of the glass bulb. Generally, the maximum value of tensile vacuum stress exists in an edge portion of a picture image displaying portion of the panel face portion or a side wall portion of the panel glass. Accordingly, if the tensile vacuum stress produced on the glass bulb surface is large and the glass bulb does not have a sufficient strength in structure, there may result a static fatigue breakage due to an atmospheric pressure and the glass bulb will not function as a cathode ray tube.
Further, in manufacturing the cathode ray tube, the glass bulb is kept at a high temperature such as about 380.degree. C. and air inside the glass bulb is evacuated. During such heating process, a thermal stress is resulted in addition to the vacuum stress. In this case, an intensive implosion is resulted due to an instantaneous introduction of air and the reaction thereof, which may damage the neighborhood.
As a guarantee to prevent such breakage of glass bulb, an external pressure loading test has been conducted. In consideration of the depth of scratches (or bruises) which may result in the surface of the glass bulb during the assembling of the cathode ray tube and the service life of it, scratches are formed uniformly in the front surface of the glass bulb by means of abrasion with a #150 emery paper, and a pneumatic pressure or a hydraulic pressure is gradually applied to the glass bulb until it causes breakage of the bulb. Then, the difference of pressure between the inside and outside of the glass bulb is measured. Generally, the glass bulb is required to have a fracture strength durable to at least 3 atm. pressure.
The fracture strength of the glass bulb with scratches is not always primarily determined because a vacuum stress in the outer surface of the glass bulb depends on the structure of it and has a two dimensional distribution as shown in FIG. 2.
FIG. 3 shows the fracture strength of various types of glass bulbs for TVs, which are made of the same material. As shown in FIG. 3, the fracture strength is 190 kg/cm.sup.2 in minimum value and about 250 kg/cm.sup.2 in average.
On the other hand, in considering the fatigue breakage of the glass bulb due to a vacuum stress, there is a high possibility of the breaking of the glass bulb from a region having the maximum tensile vacuum stress .sigma..sub.VTmax. Accordingly, in order to obtain a glass bulb for a cathode ray tube having a strength of more than 3 atm. in pressure difference between the inside and the outside of the glass bulb, which is a value for the guarantee of the pressure resistance strength, the condition of 3.0 .sigma..sub.VTmax &lt;.sigma..sub.SG should be satisfied since the linear characteristics of a elastic material can be applied to the glass bulb. Namely, since .sigma..sub.VTmax &lt;.sigma..sub.SG /3, the geometric structure such as the wall thickness, the shape and so on of the glass bulb is determined so that the maximum tensile vacuum stress .sigma..sub.VTmax is in a range of 60 kg/cm.sup.2 -90 kg/cm.sup.2 as shown in FIG. 2.
However, when the glass bulb is formed so that the maximum tensile vacuum stress .sigma..sub.VTmax is in a range of 60 kg/cm.sup.2 -90 kg/cm.sup.2, which guarantees the pressure resistance strength of the glass bulb, there is the disadvantage as follows. For instance, the weight of a panel portion for the glass bulb for a color TV cathode ray tube having an effective picture displaying surface of an aspect ratio of 4:3 (lateral direction: the longitudinal direction) is increased in proportion to a power of about 2.0-2.4 of the maximum outer dimension. Accordingly, productivity in a large-sized cathode ray tube, in particular productivity of glass bulbs is extremely reduced, and the cost of materials for the glass bulb is substantially increased.
In order to eliminate such problem, it can be considered to obtain a light-weight glass bulb by, for instance, subjecting the front surface of the glass bulb to an ion exchange treatment to thereby strengthen it. In this method, alkali ions in the glass bulb are replaced by ions larger than the alkali ions at a temperature lower than the slow cooling point of glass whereby a compressive stress is produced in the front surface of the glass bulb owing to an increased volume. For instance, such compressive stress can be obtained by immersing a SiO.sub.2 --SrO--BaO--Al.sub.2 O.sub.3 --ZnO.sub.2 series panel glass containing 5%-8% of Na.sub.2 O and 5%-9% of K.sub.2 O (5001 type glass manufactured by Asahi Glass Company Ltd.) in a melt of KNO.sub.3 kept at about 450.degree. C. for about 4-6 hrs.
With such treatment, a compression layer having a magnitude of about 1500 kg/cm.sup.2 -3000 kg/cm.sup.2 and a depth of about 10 .mu.m-30 .mu.m is formed in the front surface of the panel glass. In this strengthening method, although a layer having a large tensile stress is not formed in the glass bulb, the thickness of a compressive stress layer obtained is thin. Namely, the thickness is equal to or smaller than the depth of the scratches formed by the #150 emery paper shown in Table 1. Accordingly, a scratch penetrating the stress layer may be formed during manufacturing steps or use. In this case, the advantage of the strengthening of the panel face portion is lost.
Further, it is known to strengthen the front surface of the panel glass by using an air tempering method. In the air tempering method, the panel glass is heated to a temperature slightly lower than the glass softening point, and then, air is blasted to rapidly cool the panel glass, whereby a compressive stress layer of about 500 kg/cm.sup.2 -1000 kg/cm.sup.2 is formed in the front surface of the panel glass. In this method, the panel glass is slightly deformed after the rapid cooling treatment because the panel glass is maintained to a temperature region where glass is just softened and the surface of the glass is rapidly cooled. Accordingly, there is a problem of strengthening the panel glass for a cathode ray tube because it must have accurate dimensions. Further, a tensile stress layer is formed in the glass panel at the same time of the formation of the compression layer wherein the magnitude of the tensile stress is half as much as the absolute value of the compressive stress. Therefore, when a crack develops inside the panel glass, the panel glass implodes itself to release energy of the stored tensile stress. Accordingly, when a large tensile stress is formed in the glass bulb as a vacuum vessel of a cathode ray tube, there is a problem of an implosion.